(1) Field of the Invention
The present invention pertains to a image forming apparatus such as a printer or copier, and specifically to technology for the transportation of a recording sheet by a pair of timing rollers.
(2) Description of the Related Art
In an image forming apparatus forming images through electrophotography, for example, the transportation of a recording sheet is temporarily stopped when a leading edge of the recording sheet abuts a nip portion formed in a pair of timing rollers, which are not rotating, in order to transfer a toner image formed on a photosensitive drum to the recording sheet at a correct position. The recording sheet is then transported by starting the rotation of the timing rollers with such timing that the leading edge of the toner image (including whitespace therein) formed on the photosensitive drum and the leading edge of the recording sheet meet at a transfer position. The toner image is then transferred to the recording sheet at the correct position.
The rotation drive source for the pair of timing rollers is frequently a motor that also drives the rotation of the photosensitive drum and other components. Rotational force is transmitted from the motor to the timing rollers via a power transmission mechanism. Also, the starting and stopping of rotation by the timing rollers is controlled by the motor, via a clutch provided immediately before the timing rollers in the power transmission mechanism that is switched ON and OFF.
In recent years, the system speed has been increased in order to improve the number of images formed per unit time, and the gap for transporting the recording sheet (i.e., the paper gap) has been made smaller. Thus controlling the starting and stopping of the rotation by the timing rollers by controlling the clutch has thus become difficult to execute while still having the timing rollers handle the recording sheet with precision.
Conventional technology having a common motor driving the photosensitive drum and the pair of timing rollers has progressed by providing a motor for the timing rollers that is separate from the motor for the photosensitive drum, and removing the clutch from the power transmission mechanism. In such a system, the start and stop of rotation by the timing rollers is controlled by switching the separate motor ON (activation) and OFF (stopping).
In such a configuration, a situation may arise where, after one set of image formation operations (hereinafter, a job), a job (hereinafter, later job) using a different system speed (i.e., the transport speed for the recording sheets) than the first job (hereinafter, earlier job) is executed. A relative discrepancy was then observed to arise in terms of image formation position between the first page and subsequent pages of the later job.
Upon research into the discrepancy, the inventor has identified backlash in the power transmission mechanism as the cause. The power transmission mechanism between the motor and the timing rollers is made up of a plurality of components, such as gears, for transmitting rotational force. Each of these components has a degree of slack in the direction of rotation, or in other words, has backlash.
Thus, due to inertia, the components rotate within the range of the backlash after the motor is stopped. The rotation of the components caused by inertia after the motor is stopped is hereinafter termed momentum-driven rotation.
Accordingly, when the motor is restarted, the timing rollers begin to rotate only after the motor has rotated by an amount equivalent to the momentum-driven rotation. That is, a lag occurs between the activation of the motor and the beginning of rotation by the timing rollers, corresponding to the extent of the momentum-driven rotation (this lag is hereinafter termed rotation delay time).
The momentum-driven rotation is greater when the rotation speed of the motor is fast (i.e., when the transport speed for the recording sheets is fast), and is smaller when the rotation speed is slow (i.e., when the transport speed for the recording sheets is slow). Accordingly, the rotation delay is greater for jobs at a fast transport speed and is smaller for jobs at a slow transport speed.
As such, when, for example, an image formation job on a thick sheet at a slow transport speed is followed by an image formation job on a regular sheet at a fast transport speed, the image formation position for the first recording sheet of the later job is further upstream in the sheet transport direction than the image formation position for subsequent recording sheets.
In order to solve this problem, an approach has been devised that involves narrowing the range of tolerance for the component dimensions, so as to reduce the backlash for each component.
However, this approach reduces the yield rate of the components, and decreases manufacturability as a result of greatly reduced tolerances during assembly. Also, a certain degree of backlash between gears and the like is indispensible for the ensuring smooth rotation of engaged gears. Backlash can thus never be completely removed.
The above-discussed problem is not restricted to situations where a stop occurs at the conclusion of a job and the system speed (i.e., the transport speed for the recording sheets) is changed for a subsequent job. The problem also occurs when the transport speed for the recording sheets is changed during a single job.